1. Field of the Invention
The present invention relates; to photovoltaic cells and methods for making photovoltaic cells, and more particularly to RF sputtering for thin film cadmium telluride photovoltaic cells to improve efficiency and reduce production costs.
2. Summary of Related Art
Photovoltaic technology offers great potential as an alternative source of electrical energy. One of the main reasons that such potential has not yet been realized is the difficulty in making photovoltaic devices that efficiently transform light into electricity at a cost that is competitive with conventional energy sources. Researchers are continually striving to improve the efficiency and reduce the production costs of photovoltaic cells.
The standard photovoltaic cell includes a substrate for mounting the cell and two ohmic contacts/conductors for passing current to an external electrical circuit. The cell also includes two or three semiconductor layers in series. In addition to the n-type layer and the p-type layer of a two layer semiconductor cell, the three layer cell includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation.
The semiconductor layers may be formed from single crystalline materials, amorphous materials, or polycrystalline materials. Single crystal materials are the preferred material from an efficiency standpoint, with efficiencies over 20% available in certain single crystal photovoltaic systems. However, single crystal materials have,significant drawbacks because of the high cost of material and the difficulty in depositing single crystal materials.
In considering amorphous materials, the carrier mobility and lifetime are low. While amorphous and single crystal materials are useful in the practice of photovoltaic cells, the polycrystalline materials are preferred.
For polycrystalline materials, there are a number of advantages during the production process. Researchers in the field desire to increase the efficiency of the polycrystalline materials from the current 1-10% efficiency to the 10-15% efficiency range, which is closer to the 20-25% range of the single crystal material.
Cadmium telluride is a semiconductor whose electronic properties have long been recognized as ideally suited for photovoltaic conversion of sunlight. Cadmium telluride is preferred for thin film photovoltaic applications because of its direct band gap and its ability to be doped n-type and p-type, which permits formation of a variety of junction structures.
However, there remains a continuing need for a more efficient and less expensive photovoltaic cell, and particularly for a more efficient cadmium telluride photovoltaic cell that is suitable for mass production volume. Much of the early research work regarding cadmium telluride solar cells was restricted to the use of a single crystal cadmium telluride, which is an expensive material obtained by slowly growing a crystal from a melt containing cadmium and tellurium.
More recent photovoltaic cell development work with cadmium telluride has included methods to improve the deposition of the cadmium telluride on the substrate. Examples of deposition procedures currently available include electrodeposition, chemical vapor deposition, close spaced sublimation, solid-gas reaction, spray pyrolysis, sputtering, liquid phase epitaxy, molecular beam epitaxy, and other techniques known in the art. The present invention encompasses a new and unique application of RF sputtering for depositing both cadmium sulfide and cadmium telluride in a photovoltaic cell.
In the processes shown in U.S. Pat. Nos. 4,086,101 and 4,095,006 to Jordan et al, photovoltaic cells have been formed by coating a hot sheet of glass, previously coated with a tin oxide, with a thin film of cadmium sulfide. The film of cadmium sulfide was formed by spraying the substrate with a solution comprising a solvent, a cadmium salt, a sulphur containing compound and an aluminum containing soluble compound. The spray process is conducted while the surface of the substrate is maintained at a temperate in the range of 500.degree. F. to 1100.degree. F. After the spray process is completed, the coated substrate is heated further and then cooled to room temperature. The cadmium sulfide is converted to copper sulfide by dipping the coated glass in a weak acid solution.
A third U.S. patent to Jordan et al., U.S. Pat. No. 4,265,933 includes the additional step of irradiating the layer of polycrystallian cadmium sulfide with intense ultra-violet light during processing.
In U.S. Pat. No. 4,261,802, Fulop et al. discloses an improved electrodeposition process for producing cadmium telluride photovoltaic cells. A cadmium telluride film is formed having a barrier layer on one side and a conductive layer on the other side. The cadmium telluride is electrodeposited from either an acidic or alkaline bath onto a substrate consisting of cadmium coated on a base substrate having a rest potential more positive than that of cadmium. The cadmium telluride film is treated by annealing and plasma etching or electrolytic treatment to produce modified crystalline characteristics. The cell disclosed in Fulop et al includes only two semiconductors, a cadmium surface layer and the cadmium telluride film.
U.S. Pat. No. 4,331,707 to Muruska et al discloses an additional method for forming a thin film of cadmium sulfide. The film of cadmium sulfide is fabricated by first depositing a layer of cadmium oxide by spray pyrolysis of a cadmium salt solution followed by annealing of the oxide layer at elevated temperature in the presence of a sulfide ion. The thermally induced ion exchange of the sulfur for oxygen produces a cadmium sulfide.
U.S. Pat. Nos. 4,388,483; 4,666,569; and 4,680,611 to Basol et al disclose an alternative means for electrodepositing a layer of semiconductor material and heating the film to create a low-resistivity p-type semiconductor compound. Also disclosed is a multi-layer ohmic contact for the p-type semiconductor. The Ohmic contacts have two metal layers, a first layer to provide a stable contact with the p-type semiconductor and a second layer to provide the electrical conductivity. Additional patents which disclose a process for fabricating thin film photovoltaic cells include U.S. Pat. Nos. 4,400,244 to Kroger et al for electrochemical deposition; 4,709,466 to McCandless for heat treating and then rapid cooling of the photovoltaic cell; and 4,734,381 to Mitchell for lead telluride deposited on cadmium telluride.
Several recent patents have disclosed p-i-n photovoltaic cells with three layers of semiconductors. The advantages of using three layers of semiconductors instead of two layers of semiconductors include the ability to choose materials with properties for each component of the device. Solar radiation is absorbed in the intrinsic layer, and photogenerated positive and negative charge carriers are field assisted to the p and n regions. Cadmium telluride is self-compensating and prefers to be intrinsic. The n-type layer generally consists of cadmium sulfide and the p-type layer generally consists of zinc telluride.
U.S. Pat. No. 4,753,684 Ondris et al. teaches a three semiconductor layer structure prepared by electrodeposition. The process disclosed includes three electrodeposition baths with process temperatures less than 100 degrees Celsius and with limited exposure time between the three baths. U.S. Pat. No. 4,710,589 to Meyers et al teaches a photovoltaic cell having an n-type layer of cadmium sulfide deposited by vacuum evaporation or a narrow gap reactor, an intrinsic layer of electrodeposited cadmium tellurium, and a p-type layer of zinc tellurium deposited by vacuum evaporation.
DC and RF sputtering are widely used techniques for the deposition of thin films for insulators and metals. However, the use of sputtering techniques for photovoltaic cells has not tested or utilized.
In most RF sputtering systems a cathode and an anode are positioned in an atmosphere of heavy gas, such as argon. A radio frequency field is established between the cathode and the anode for the sputtering of materials. The cathode is the target and is made up of the materials to be sputtered. The anode is the electrode toward which the molecules of the target are driven for deposit. In planar magnetron sputtering, the anode consists of a ring-shaped electrode with diameter slightly larger than the circular target. Permanent magnets are placed under the target, thereby creating a magnetic field which serves to constrain the electron and ion paths. When the RF voltage is applied between the target (typically negative) and the anode ring (typically grounded positive), the electrons are concentrated in a circular orbit a few millimeters above the target. This type of magnetron sputtering gun has the advantage of minimizing the bombardment of the substrate with electrons and ions, often improving the film characteristics.
The frequencies at which sputtering machines may operate are established by the Federal Communications Commission. The frequencies include 13.56 megahertz and the second and third harmonics of that frequency.
Large sputtering machines are not limited in processing to the size of the pressure chamber. Air or pressure locks in the walls of the pressure chamber enable continuous sputtering upon a pass-through substrate without opening the chamber.
U.S. Pat. No. 5,105,310 to Dickey discloses a method for depositing a multi-layer antireflection coating on a transparent substrate by DC sputtering. DC sputtering is often used for large scale commercial applications. In line vacuum chambers are provided for multiple layers.
A process for the sputter deposition of dielectric materials, such as aluminum oxide, is disclosed in U.S. Pat. No. 4,420,385 to Hartsough. U.S. Pat. No. 4,392,931 to Maniv et al also teaches a sputtering system designed primarily for oxides.
U.S. Pat. No. 4,415,427 to Hidler teaches a planar magnetron sputtering means having a defined plasma area. The host target material and a dopant material are disposed in the magnetron area and are deposited at the same time.
It is often difficult with the known RF sputtering systems to obtain a thin film with controlled resistivity, transparency, and composition for use in semiconductor applications. U.S. Pat. No. 4,398,055 to Ijaz et al discloses the sputtering of a photovoltaic layer of a polycrystalline film consisting of cadmium, silicon, and arsenide.
U.S. Pat. No. 4,284,490 to Weber provides details regarding the equipment and processes that are utilized to sputter at a RF in the megahertz range.